1. Field of the Invention
The present invention relates to a distance detecting method suitable for detecting a relative distance between mobile stations or between a mobile station and a base station, and its apparatus, and particularly to a distance detecting method applicable to a mobile communication system using a spread spectrum communication system and its apparatus.
2. Description of the Related Art
In recent years, a detecting system for detecting a relative distance between two mobile stations using a spread spectrum communication system has been developed. For example, in an inter-vehicle communication apparatus described in Japanese Patent Application Unexamined Publication No. Hei-5-122120, a certain communication station (self-vehicle) MS-1 radio transmits a spectrum spread signal to the other communication station (other vehicle) MS-2, and the other communication station MS-2 receives the spectrum spread signal from the communication station M-1. Then, the other communication station M-1 modulates the transmission data using spreading code and sends the spread signal back to the communication station MS-1. The certain communication station MS-1 receives that response from the other communication station MS-2. Then, the communication MS-1 detects a time difference between the transmission of the transmitted signal sent from the communication station MS-1 and the received signal sent from the other communication station MS-2, and a relative distance between two communication stations is obtained by the following equation:
Relative distance=velocity of lightxc3x97time difference/2
The above-mentioned inter-vehicle communication apparatus is predicated on the following point.
More specifically, the receiver station receives the transmission signal from the sender station. At this time, the receiver station performs spreading modulation. In this spreading modulation, the response data is spread by a spreading code whose head substantially coincidence with a reception timing, and transmits the spread signal to the sender station. In other word, it is required that the receiver station generates the spreading code in synchronization with the reception sgnal and sends back the response signal. For this reason, it is difficult to insert various data for other than the detected distance to the transmission signal in CDMA system.
Also, the sender station performs the distance detection based on the premise that the receiver station sends back the response signal spread by the spreading code synchronized with the reception signal. For this reason, the synchronous accuracy in the receiver station has a direct influence upon the accuracy of the distance detection.
In consideration of the above problem, the present invention has been made, and an object of the present invention is to provide a distance detecting method and its apparatus wherein a sender station and a receiver station can receive and transmit a signal asynchronously by use of a spreading code generated at a timer of each station, other information can be simultaneously transmitted and a distance detection having high reliability can be performed without exerting influence upon synchronous accuracy of the receiver station.
In the distance detection method of the present invention, a signal having periodicity is transmitted to a communication station B based on a reference timing generated by a timer which a communication station A has, while the communication station B, which has received the transmitting signal of the communication station A, transmits a signal, which is generated based on a reference timing generated in its internal section and receives the signal. Then, a phase difference between the transmitting signal and the receiving signal is detected so as to obtain a distance between the communications stations A and B.
According to the present invention, the distance detection is carried out by the phase difference including the difference of the reference timing between the communication stations and propagation delay time, each communication station may receive and transmit the signal asynchronously with the reception timing at the timer of the self-station. As a result, the distance detection can be easily carried out without exerting a large influence upon communications of other information, and high detection accuracy can be realized without exerting influence upon synchronous accuracy of the target station.
Also, in the distance detecting method of the present invention, a signal having periodicity is generated based on the timer of the self-station and is transmitted to the target station, while the self-station receives a signal, which is generated based on the timer of the target station and transmitted therefrom. Then, by use of the phase difference between the transmitting signal transmitted from the self-station to the target station and the receiving signal received from the target station, the relative distance between the self-station and the target station is detected.
According to the present invention, the distance detection is carried out by the phase difference including the difference of the reference and propagation delay time, each of the self-station and the target station may receive and transmit the signal asynchronously with the reception timing based on the timer of each station. As a result, the distance detection can be easily carried out without exerting a large influence upon communications of other information, and high detection accuracy can be realized without exerting influence upon synchronous accuracy of the target station.
Also, in the distance detecting method and its apparatus of the present invention, the receiving signal received from the target station is demodulated so as to obtain a target station detecting phase difference. Then, by use of the target station detecting phase difference and a self-station detecting phase difference, the relative distance between the self-station and the target station is detected based on the following equation:
Distance=Kxc3x97(target station detecting phase difference+self-station detecting phase difference)/2
where K is a constant corresponding to velocity of light.
According to the above-mentioned configuration, since the distance detection can be carried out by use of the phase difference detected by the target station and the phase difference detected by the self-station, each of the self-station and the target station can receive and transmit the signal asynchronously with the reception timing based on the timer of each station.
Also, in the distance detecting method and its apparatus of the present invention, the receiving signal received from the target station is demodulated so as to obtain the target station detecting phase difference. Then, by use of the target station detecting phase difference and the self-station detecting phase difference, the difference of the reference timing between the timer of the self-station and the timer of the target station is detected.
According to the above-mentioned configuration, the difference of the reference timing between the timer of the self-station and the timer of the target station can be detected by use of the target station detecting phase difference and the self-station detecting phase difference. As a result, the timer shift between the self-station and the target station can be easily corrected.
Also, in the distance detecting method and its apparatus of the present invention, the receiving signal is synchronized with the reference timing of the timer of the target station and is spread modulated. Then, a despreading code is synchronized with the reference timing of the timer of the self-station and is relatively shifted. A correlation value between the receiving signal and the despreading code is detected. The self-station detecting phase difference is obtained based on the number of shifts showing a maximum correlation value and a shift amount per one time.
According to the above-mentioned configuration, the self-station detecting phase difference can be easily obtained by use of a sliding correlator in performing a spectrum spread communication.